For drip infusion or direct blood pressure measurement, an infusion line or a catheter is usually inserted into a patient's vein or artery so as to sample blood, for example for examination purposes, from an externally accessible location of the infusion line. To this end, for example, taps or, in relatively recent systems, sampling sites may be provided which communicate with an interior lumen of the infusion system and are accessible from outside by means of a hypodermic syringe.
Before blood can be sampled from this system, one should first of all ensure that only undiluted blood of the patient without any constituents of infusion solutions or anticoagulants are located in the area of the sampling site and/or at the tap, said anticoagulants being supplied, for example, in direct blood pressure measurement to prevent coagulation of the blood. To this end, for example, WO 88/01846 A1 envisages two sampling sites; the sampling site located closer to the patient serves to actually draw the blood sample. The sampling site located further away from the patient serves to temporarily remove infusion solutions from the system so that only undiluted blood will be present at the upstream sampling site. WO 88/01846 A1 thus offers an improved possibility of blood sampling as compared to an approach wherein a sodium chloride solution and mixed blood is initially drawn at a tap by using a syringe until pure blood can be sampled at the tap. Subsequently, blood for laboratory analysis (e.g. for examining blood gases, etc.) is sampled at this tap by means of a second syringe. The initially drawn amount of infusion solution and blood should not be recirculated to the patient since, due to the syringe aspiration process, blood cells are damaged and contamination of the blood may occur. When blood sampling is performed frequently, this drawing of mixed blood prior to each blood sampling without subsequent recirculation leads to a noticeable loss of liquid.
In the implementation according to WO 88/01846 it is suggested, despite the risk of contamination and cell damage, to recirculate the amount which has been drawn at the downstream sampling site by means of a conventional syringe, the needle of which penetrates a sealing plug which is otherwise tight, to the system. However, this involves extensive handling and involves the risk—in addition to the risk that contaminants, germs and other pathogens are introduced into the system—that the hospital personnel might be injured and infected by the needle while removing same, which has already led to AIDS or hepatitis infections in hospital personnel before.
It has therefore been proposed to use a system which is closed in that the downstream sampling site is configured as a temporary storage which is integrated in the pressure measurement or catheter system and has a piston/cylinder arrangement no longer accessible from outside, as is reported, for example, in the introduction to the description of EP 0575917 A2. However, with said blood sampling device, the problem arises that the temporary storage is not fully emptied in the event of reinfusion of the patient's blood, so that residual blood will coagulate there. If the above-mentioned processes are performed several times, it may therefore occur that coagulated residual blood returns to the patient's blood circulation from the temporary storage and highly endangers said patient.
Therefore, EP 0575917 A2 proposes a conical piston tip in connection with a conical cylinder space tip, the cone angle of the piston tip being larger than that of the cylinder space tip so as to ensure complete emptying during squeezing out. However, it becomes apparent that even with such a provision within a closed blood sampling system attached to a patient over a relatively long period (e.g. several days), it cannot be ensured that the inner space of the blood sampling system, in particular of the temporary storage, is not contaminated by introduced germs. Such introduction of germs may occur, for example, when the piston is actuated several times.
In addition, conventional approaches to blood sampling by means of a temporary storage have the disadvantage that in many cases it cannot be ensured that blood and/or infusion solution is not aspirated out of the patient and/or the pressure measurement line at too high a negative pressure. Too high a negative pressure caused by, e.g., excessive pulling of the piston may result in that degassing occurs which involves corresponding bubble formation taking place in the blood, and that the blood-vessel wall collapses and/or that vascular obstruction occurs, which may result in necrotizing of tissue right up to a patient's death, depending on the arterial sampling site in the patient.
Even though excessive negative pressure formation may be prevented by appropriately cautious handling on the part of the hospital personnel when manipulating the temporary reservoir, this involves a correspondingly high standard of training and knowledge on the part of the personnel. Notwithstanding the above, the process involves a large amount of time on account of the care that is taken.
Finally, a temporary storage of a closed blood system makes sterilization by means of, e.g., ETO gas more difficult in a state wherein the temporary storage is in a state in which it is assembled and already arranged, e.g., in a gas-permeable packaging.